FAN8620B Datasheet, PDF (17/35 Pages) Fairchild Semiconductor – 12V Spindle Motor and Voice Coil Motor Driver

FAN8620B
Application Information
Spindle Motor Drive Circuit
The FAN8620B is a combination chip consisting of spindle motor and voice coil motor designed for HDD system. According
to the spindle conditions, the digital ASIC provides optimum control signals (Start-up, commutation, speed control, and com-
mutation mode) to the FAN8620B.
Back-EMF (BEMF) signal of the spindle motor is fed back to ASIC via FG line. The MCLK and PWM signals are used to
determine the commutation timing and to control the spindle speed, respectively.
Spindle Driver
The spindle includes both low and high side drivers (H-bridge) for a three-phase sensorless brushless DC motor. To reduce the
saturation voltage, the vertical PNP transistor is used as the high side driver.
Frequency Generation (FG)
FG stands for Frequency Generation. It is the output signal to the ASIC.
It contains important information about the motor speed.
According to the FG frequency, the digital ASIC provides different motor clock signals to the motor drive IC via MCLK.
It checks the motor speed to send the VCM enable signal via CNTL3.
FG frequency (Hz), motor speed (rpm) and pole number are directly related as shown below in the three phase motor.
FG frequency(Hz) = motor speed(rpm) × pole number / 2 × 3 / 60
In a typical application,(8 pole motor)
FG frequency = 5400 × 8 /2 × 3 / 60 = 1080 [Hz]
MCLK & Mask
The MCLK is a motor clock used as the standard clock signal for the proper commutation timing of the spindle motor. It is
supplied by the ASIC.
As shown in table 1, it has different delay times depending on the mode of the spindle.
Start-up mode
Acceleration mode
Running mode
MCLK (Td)
2ms (External ASIC)
FG(n-1) / 2
FG(n-1) / 32
MASK
1ms
FG(n-1) / 4
344.45ms
Commutation
Hard
Hard
Soft
Table 1.
After the FG_Edge signal detection, the MCLK occurs after a half FG_Edge delay time in the acceleration mode and 1/ 32
FG_Edge delay time in the soft commutation mode.
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